The present invention generally relates to the manufacture of contact lenses and more specifically relates to methods for cast molding contact lens with a molded edge that requires no dedicated post-processing steps.
Contact lenses have historically been made by machining a lens material, in button (or block) form on front and back surfaces thereof to produce an unfinished lens product having the required fit, or “base curve”, and visual correction, to compensate for one or more refractive abnormalities of the eye. Such refractive abnormalities may include myopia (nearsightedness), hypermetropia (farsightedness), astigmatism, presbyopia and the like. Using conventional machining technology, the optic faces of the unfinished lens require polishing in order to remove rings, known as “turning rings”, on the unfinished lens that have been created by the machining process. Typically the edge of the lens is also polished to produce smooth edge geometry, or profile, in order to maximize wearer comfort and prevent injury to the eye. The edge polishing step is particularly important in the manufacture of “hard” lenses, in which the lens material is relatively rigid and inflexible and even small defects on the edge can cause irritation in the wearers' eye.
The machining process described above, being very labor intensive, has been widely replaced by cast molding of the lens. In conventional cast molding techniques, front and back surface mold halves are made for the required lens prescription. Each mold section is produced by injection molding techniques. A mold insert tool, for example made of metal, ceramic or the like, is provided and secured or fixed in an injection molding machine. A mold section, or mold half, is then produced having the contour of the insert. Thus, the mold section may include a negative impression of, or a contour which corresponds to, the desired lens front or back surface.
The two mold halves are assembled and lens material, for example a curable, mixture of polymerizable monomers, is introduced between the assembled mold halves. After the cast molding steps, subsequent processing steps are employed, which may include for example, curing, demolding, and hydration (in the case of hydrogel lenses). In addition, edge polishing operations are often performed to smooth the edge profile of the lens.
A method of cast molding contact lenses, particularly toric contact lenses, is described in Appollonio, et al, U.S. Pat. No. 5,611,970 which is incorporated in its entirety herein by reference.
Methods for fully cast molding contact lenses have been developed. Fully cast molded lenses are lenses in which all surfaces, front, back and edge of the lens are molded as part of a mold pair and no post processing polishing or machining operations are performed to yield the final lens product.
Unfortunately, although fully molded lenses provide an inexpensive and flexible alternative to machined and polished cast molded lenses, fully molded lenses are sometimes uncomfortable for the wearer and may even harm the eye, particularly after long term use.
It has been shown that the form and size of the edge of a contact lens have significant effects on wearer comfort and conjunctival health. It is conventional that a fully molded lens will have a lens edge form which is “chiseled”, or “pointed” in some way, and is not representative of a polished edge or an edge which had been historically manufactured to improve wearer comfort.
Wearers of conventional fully molded lenses have been known to suffer from eye irritation and reduction in “end of day comfort” of the eye. Moreover, long term wearers have been observed, with the use of a fluorescein dye, to have ‘scuff’ marks formed on the sclera, the scuff marks being caused by interaction between the lens and eye. To achieve a more desirable edge geometry and provide a more comfortable lens, some lens manufacturers will apply additional, separate, post processing steps such as edge polishing, in otherwise fully molded lenses. Polishing the cast molded lens will generally remove the chiseled or pointed edge profile that is conventionally produced by cast molding. Understandably, however, the additional manufacturing step will tend to increase the cost of the final lens product.
It would be highly advantageous to provide a method by which a lens could be cast molded, the resulting lens having a desirable, substantially smooth or rounded edge geometry, without the need for post processing polishing steps.